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Vol 42: Human AP endonuclease inefficiently removes abasic sites within G4 structures compared to duplex DNA.This article is from Nucleic Acids Research, volume 42.AbstractExcision repair processes are essential to maintain genome stability. A decrease in efficiency and fidelity of these pathways at regions of the genome that can assume non-canonical DNA structures has been proposed as a possible mechanism to explain the increased mutagenesis and consequent diseased state frequently associated with these sites. Here we describe the development of a FRET-based approach to monitor the presence of G quadruplex (G4) DNA, a non-canonical DNA structure formed in runs of guanines, in damage-containing single-stranded and double-stranded DNA. Using this approach, we directly show for the first time that the presence within the G4 structure of an abasic site, the most common lesion spontaneously generated during cellular metabolism, decreases the efficiency of human AP endonuclease activity and that this effect is mostly the result of a decreased enzymatic activity and not of decreased binding of the enzyme to the damaged site. This approach can be generally applied to dissecting the biochemistry of DNA repair at non-canonical DNA structures.